1. Field of the Invention
The present invention relates to a scanning exposure method and a scanning type exposure apparatus, and more particularly to a scanning exposure method used when manufacturing semiconductor elements, liquid crystal display elements, thin-film magnetic head, or the like by a lithographic process, and to a scanning type exposure apparatus applicable to this exposure method.
This application is based on Japanese Patent Application No. Hei 9-339349, the contents of which are incorporated herein by reference.
2. Background Art
Heretofore, with the lithographic process for manufacturing semiconductor elements or liquid crystal display elements or the like, a projection exposure apparatus is used which transfers a pattern formed on a mask or reticle (referred to hereunder in general as a reticle) onto a substrate of for example a wafer or glass plate which has been painted with a photoresist or the like (suitably referred to hereunder as a sensitive substrate or a wafer), by means of a projection optical system. For this type of apparatus, for example in the manufacturing process for semiconductor elements, heretofore there has principally been used a reduction projection type exposure apparatus of a stationary exposure type (also referred to as a step-and-repeat system) which repeatedly switches between a stepping operation for positioning a wafer stage on which is mounted a wafer serving as a sensitive substrate at a predetermined exposure position by moving a predetermined amount in the two dimensional X, Y direction, and an exposure operation where while positioned, a pattern of a reticle is transferred to a shot area on the sensitive substrate by means of a projection optical system. However with the yearly increase in the amount of integration of semiconductors and the accompanying increase in detail of circuit patterns, then with the performance of exposure apparatus, there is the requirement for even higher resolving powers, and increased exposure accuracy. As an exposure apparatus which addresses these requirement, recently a scanning type exposure apparatus, for example a step-and-scan type scanning type exposure apparatus has gained attention. With this apparatus, a reticle stage holding a reticle, and a wafer stage are simultaneously moved in a predetermined scanning direction with respect to a projection optical system while illuminating a predetermined slit shaped area on the reticle with illumination light. As a result patterns inside the slit area on the reticle are transferred one by one onto the wafer by mean of a projection optical system, thereby transferring the reduction image of the whole surface of the reticle pattern onto a shot area of the wafer.
With such a scanning type exposure apparatus, from the view point of improving throughput, it is general to adopt a scanning exposure method in which successive reticle patterns are transferred to a plurality of shot areas on the wafer while reciprocally moving the reticle stage in the scanning direction.
Furthermore, with the conventional scanning type exposure apparatus, the position of a best image formation surface of the projection optical system is measured beforehand using a predetermined sensor, without moving the reticle and the wafer stage in a synchronous manner, and focus levelling control during scanning exposure is performed using a focus sensor, in order to make the area to be exposed of the wafer surface (the slit shape area on the wafer corresponding to the illuminated area on the reticle) coincide with the position of the best image formation surface (more accurately, within the focal depth range of the best image formation surface).
With the conventional scanning type exposure apparatus, as described above the position of the best image formation surface is fixed as the target position in the optical axis direction of the projection optical system with which the area to be exposed of the wafer surface must be aligned, and focus levelling control is performed during scanning exposure.
However, with the scanning type exposure apparatus, it has recently become apparent that because in order to perform scanning exposure while synchronously moving the reticle and the wafer, focus control is performed with a certain position of the best image formation surface measured as a target value, then it is not always possible to make this perfectly coincide with the target surface for aligning the area to be exposed of the wafer. This is presumably attributable to inclination (deformation) attributed to the occurrence of vibration or movement of the centre of gravity position in the exposure apparatus body during synchronous movement, to the limits of the control response at the time of focusing due to mechanical performance of the stage drive system, and to the limits in alignment accuracy.
According to information of the present inventor obtained from the results of research, the amount of this non-coincidence differs for each apparatus. Furthermore this also differs due to the movement conditions of both stages, such as the movement direction of the reticle or the wafer or the positional change of the reticle (reticle stage) and the wafer (wafer stage) during synchronous movement, and the movement speed of the reticle stage and the wafer stage.
With the exposure accuracy obtained heretofore, the focus error arising due to the abovementioned sources is of a degree which can be disregarded. However with future requirements for exposure accuracy, it is expected that these will gradually become more stringent. Hence development of a scanning exposure method and scanning type exposure apparatus which can reduce the focus error and levelling error attributable to the movement conditions of the wafer stage or the reticle stage as described above, has now become a pressing need.